This invention generally relates to the field of power interface circuits and, more particularly, to power interface circuits for time division multiple access (TDMA) transmitters.
TDMA communication systems have been widely used in today's digital cellular systems, to communicate voice and data between a plurality of portable or mobile transceiver units. In such systems, a radio frequency channel is subdivided into a number of TDMA channels comprising time slots during which the transceiver units communicate TDMA messages with each other. Each transceiver unit includes a TDMA transmitter that transmits bursts of messages during an assigned transmit channel and a TDMA receiver that receives the messages during an assigned receive channel.
In order to minimize battery current drain, a critical resource in the portable communication units, certain components of the TDMA transmitter and receiver may be energized only during their assigned channels. Moreover, even during the assigned channels, transmitter components may be turned off during periods of silence to still further reduce consumption. For example, each transmitter includes a radio frequency power amplifier (RFPA), which draws relatively significant battery current when energized. Thus, the transmitter is equipped with a power interface circuit that selectively couples the supply voltage to the RFPA only during an assigned transmit channel, to minimize battery current drain. Generally, such power interface circuit includes a switch that connects and disconnects the supply voltage to and from the RFPA under the control of a switch control signal.
Conventionally, p-channel metal-oxide semiconductor field effect transistors (MOSFETs) have been used as switches in the power interface circuits. FIG. 1 shows schematic diagram of one conventional power interface circuit 10 that uses such p-channel MOSFET 12, which is biased through resistors 11 and 13 to selectively connect and disconnect a supply voltage provided on line 14 to an RFPA 16. In response to a voltage supply control signal on line 18, which has two binary states, the p-channel MOSFET is turned on to connect the supply voltage to the RFPA 16 during one binary state and is turned off to disconnect the supply voltage from the RFPA 16 during another binary state.
To turn on the p-channel MOSFET 12, a negative voltage is applied across the transistor's gate-to-source junction that increases (or enhances) the conductivity of its p-channel. Under the arrangement of FIG. 1, a negative gate voltage, i.e. ground, is applied to the p-channel MOSFET, through a bipolar transistor 20, to turn on the p-channel MOSFET, and a positive gate voltage, i.e. V.sub.SUPPLY, is applied to turn it off.
Because of the desire to minimize the size and cost of portable communication units and increase their transmission speed, manufacturers are continuously searching for ways to reduce the size of the circuitry used in such units, while increasing their switching speed. Compared to p-channel MOSFETs, n-channel MOSFETs occupy two to three times less die space and provide faster switching speeds. They could also be configured to protect the RFPA from voltage peaks through simple regulation circuits, for example, by coupling a Zener diode at the gate of the MOSFET. Thus, the n-channel MOSFETs are an attractive alternative to p-channel MOSFETs in communication system switching applications.
Despite the advantages offered by the n-channel MOSFETs, their use in portable communication units have been limited because in order to use the n-channel MOSFET as a switch, the MOSFET must be biased to operate in an on-state. Biasing the n-channel MOSFET in the on-state requires application of a gate voltage that is more positive than the supply voltage; a task not easily achievable given the supply voltage is the highest voltage available in the communication unit.
Thus, there exists a need for a power interface circuit that can bias an n-channel MOSFET to provide selective switching between the supply voltage and the RFPA of a TDMA transmitter. The present invention addresses this need.